A fuel cell is an electrochemical cell comprising two electrodes separated by an electrolyte. A fuel, such as hydrogen or an alcohol, such as methanol or ethanol, is supplied to the anode and an oxidant, such as oxygen or air, is supplied to the cathode. Electrochemical reactions occur at the electrodes, and the chemical energy of the fuel and the oxidant is converted to electrical energy and heat. Electrocatalysts are used to promote the electrochemical oxidation of the fuel at the anode and the electrochemical reduction of oxygen at the cathode.
In the hydrogen-fueled or alcohol-fueled proton exchange membrane fuel cells (PEMFC), the electrolyte is a solid polymeric membrane, which is electronically insulating and proton conducting. Protons, produced at the anode, are transported across the membrane to the cathode, where they combine with oxygen to form water. The most widely used alcohol fuel is methanol, and this variant of the PEMFC is often referred to as a direct methanol fuel cell (DMFC).
The principal component of the PEMFC is known as a membrane electrode assembly (MEA) and is essentially composed of five layers. The central layer is the polymeric ion-conducting membrane. On either side of the ion-conducting membrane there is an electrocatalyst layer, containing an electrocatalyst designed for the specific electrocatalytic reaction. Finally, adjacent to each electrocatalyst layer there is a gas diffusion layer. The gas diffusion layer must allow the reactants to reach the electrocatalyst layer and must conduct the electric current that is generated by the electrochemical reactions. Therefore the gas diffusion layer must be porous and electrically conducting.
The electrocatalyst layers are generally composed of a metal, (such as a platinum group metal (platinum, palladium, rhodium, ruthenium, iridium and osmium), gold or silver, or a base metal) either unsupported in the form of a finely dispersed metal powder (a metal black) or supported on an electrically conducting support, such as a high surface area carbon material. Suitable carbons typically include those from the carbon black family, such as oil furnace blacks, extra-conductive blacks, acetylene blacks and graphitised versions thereof. Exemplary carbons include Akzo Nobel Ketjen EC300J and Cabot Vulcan XC72R. The electrocatalyst layers suitably comprise other components, such as ion-conducting polymer, which is included to improve the ionic conductivity within the layer. The electrocatalyst layers also comprise a certain volume fraction of porosity, which allows reactant ingress and product egress.
Conventionally, the MEA can be constructed by a number of methods outlined hereinafter:
(i) The electrocatalyst layer may be applied to the gas diffusion layer to form a gas diffusion electrode. Two gas diffusion electrodes can be placed either side of an ion-conducting membrane and laminated together to form the five-layer MEA;
(ii) The electrocatalyst layer may be applied to both faces of the ion-conducting membrane to form a catalysed ion-conducting membrane, either by direct application or indirectly by transfer from a previously coated decal transfer sheet. Subsequently, gas diffusion layers are applied to both faces of the catalysed ion-conducting membrane.
(iii) An MEA can be formed from an ion-conducting membrane coated on one side with an electrocatalyst layer, a gas diffusion layer adjacent to that electrocatalyst layer, and a gas diffusion electrode on the other side of the ion-conducting membrane.
Typically tens or hundreds of MEAs are required to provide enough power for most applications, so multiple MEAs are assembled to make up a fuel cell stack. Field flow plates are used to separate the MEAs. The plates perform several functions: supplying the reactants to the MEAs; removing products; providing electrical connections; and providing physical support.